The areas of sub-tropical high pressure are located between latitudes 25″ and 35″ N and S. The most important feature of this pressure belt is that it is broken into a number of high pressure centers or cells.
These high pressure cells or ‘centers of action’ are the key points in the distributional pattern of air pressure over the globe. But the exact causes of their origin are not yet fully known. The meteorologists agree that these subtropical high pressure cells are dynamically produced due to the rotation of earth.
The subtropical highs are areas of sinking and settling air from higher altitudes which tend to build up atmospheric pressure. In the upper atmosphere over this belt the upper level westerlies and anti-trades converge and set up descending currents in the atmosphere.
These high pressure areas have no prevailing winds. The winds are light and variable. There are occasional calms. However, sometimes these belts are invaded by extra-tropical or tropical disturbances attended by stormy winds.
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Because of the subsidence of air in these areas, the weather for most part of the year remains fair and dry. Subsidence also causes an increase in the temperature of surface air. All the hot deserts of the world lie in this belt, and are located on the western margins of the continents.
These high-pressure zones are called the horse-latitudes. The cells of high pressure persist throughout the year over the large ocean basin where there is a slight seasonal change in their position. However, there are marked variations in their intensity and size.
In the southern hemisphere higher pressures are observed during the winter seasons. In this hemisphere there is an almost continuous belt of high pressure, because the high pressure cells spread to adjacent continental areas during the winter season.
In the northern hemisphere, during the warmer part of the year, the subtropical high-pressure areas record higher pressures and become larger in area. Even though they are connected with the continental highs at higher latitudes by the ridges of high pressure, the continuity of the belt is not maintained.
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The daily weather charts do not confirm the semi-permanent character of the subtropical high pressure belts because of frequent variations in their size, location and intensity. The subsiding air currents produce stability in the atmosphere and also create dry air masses.
Rainfall is extremely limited in areas under the influence of high pressure cells. Another characteristic feature of subtropical highs is that they extend to about 9,600 meters above the mean sea-level.
As discussed earlier, most of the subtropical anticyclones are asymmetrical in their internal structure. In close proximity to the earth’s surface the centre of the highest pressure is located over eastern part of the oceans, while at greater elevations the same is located farther to the west.
The axis, therefore, slopes gently upwards towards the west. This asymmetry in the internal structure results in the preponderance of very stable air masses in the eastern half of these high pressure cells. However, the western half is distinguished by more unstable and humid air masses.